13th International Conference on Fracture June 16–21, 2013, Beijing, China According specimen designs, the specimen center has zero axial displacement theoretically during ultrasonic fatigue loading. The displacement rises continually at position both side away from the center. There is a small area on the surface which includes specimen center remains visible by optical microscope because the displacement maintained at very low level. Therefore a mobile optical microscope constructed for taking the online image. It has magnification 320X and recording image at frequency up to 1 Hz. With this mobile optical microscope, it is possible to observe general appearance when character deformation takes place in the specimen surface like enforced slip bands. The typical online observation system is shown at Fig. 3. Fig. 3 Experimental system 3. Results and discussions By the advantage of displacement node and stress distribution at center of gigacycle fatigue specimen, with online optical microscope observation, Fig. 4 shows the evolution of irreversible deformation in the specimen surface by increasing number of cycles. It is found that, irreversible slip bands formed at the beginning of fatigue loading. This slip bands are multiplied or grown in the same grain due to following cycle loading. At neighbor area, new irreversible slip bands formed independently in another grain where no deformation appears at previous 1.6x107 cycles. Grain, either one already with slip band or one without slip band can has new slip band occurrence due to fatigue loading in same test condition. At the slip bands site where optical observation carried out, most of irreversible slip bands formed before 3x108 cycles. Only slightly evolution occurs by cyclic loading during fatigue life between 3x108 and 1010 cycles. Fig.4 suggests that new irreversible deformation (localized plastic strain) per each loading cycle Δε/N is decreasing with increasing number of cycle. Δε/N closing to zero after 3x108 cycles in this case. In other words, crack initiation due to cumulating further new irreversible deformation after 3x108 cycles is scattered dispersed. That may be the reason why the S-N curve is normally significantly dispersed at regime beyond 109 cycles.
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